Scratch is in the process of a HTML5 rewrite with the help of Google, which makes sense given that it's the key component of their CS First program.

https://wiki.scratch.mit.edu/wiki/Scratch_3.0

Sounds like it should be ready by 2020!

Honestly, I need it now. I tried to pull up Scratch for my child, and the "create" link went to a blank page without even an error message. Given that no browsers on my platform still support NSAPI plugins, there is literally no way for my child to use Scratch, and they are the perfect age for it.